1. Field of the Invention
The present invention relates to Turbo decoding of digital data transmission received over a noisy channel, and particularly to use of scaled feedback in Turbo decoding.
2. Description of the Related Art
The paper xe2x80x9cA Mathematical Theory of Communicationxe2x80x9d (C. E. Shannon, Bell System Technical Journal, 27:349-423, 623-656, October, 1948) set forth xe2x80x9cThe Noisy Channel Coding Theoremxe2x80x9d, which stated that so long as the rate at which information is transmitted over a channel is less than the channel capacity, there exist error control codes that can provide arbitrarily high levels of reliability at the receiver output. However, the paper did not provide any actual coding methods for achieving arbitrarily high reliability.
The effectiveness of a code is usually expressed in terms of coding gain; i.e., the difference between the Eb/No (energy per bit over noise, a signal-to-noise ratio) required to achieve a given BER (bit error rate) in the coded system and the Eb/No required to achieve the same BER without benefit of coding.
Improvements in coding gain were made slowly. An early factor was the introduction of Golay code in the early 1950""s, followed by some NASA-inspired uses of Reed-Solomon Code and Viterbi Code in the 1980""s. Some of these implementations required very complex hardware, and yet, forty-five years after the publication of Shannon""s paper, a gap of almost 2 db. continued to separate the performance of the most advanced error-control systems from the theoretical limit (the xe2x80x9cShannon limitxe2x80x9d).
A new coding method was announced in the paper xe2x80x9cNear-Shannon-Limit Error-Correcting Coding and Decoding: Turbo Codesxe2x80x9d (C. Berrou, A. Glavieux, and P. Thitmajshima, Proceedings of the 1993 International Conference on Cominunications, pages 1064-1070, 1993). (See generally Turbo Coding, Heegard and Wicker, Kluwer Academic Publishers, Norwell, Mass., 1999, ISBN 0-7923-8378-8.)
Although Turbo Codes have effected significant advances in coding gain, additional advances, closing even further the gap with the Shannon limit, are still desirable. In space communication, for example, transmitter power is inherently limited; in terrestrial wireless communication, a steep increase in the kinds of services available and in the demand for them is making bandwidth scarce. It has been estimated in the Deep Space Project that one db. of coding gain is worth eighty million dollars in spacecraft equipment costs (Turbo Coding at page 5).
The Turbo decoder of the present invention has improved coding gain over the prior-art Turbo decoders.
The invention is an improvement to iterative Turbo decoders of the prior art. In a prior-art Turbo decoder, a Turbo-coded signal is repetitively subjected to the process of decoding in a first decoder, interleaving, decoding in a second decoder, and deinterleaving. The improvement of the present invention is to scale the signal after each decoding step by a predetermined value. Predetermined values in the vicinity of 0.7 to 0.8 are found to improve coding gain.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.